1. Field of the Invention
The present invention relates to a vertical structure light emitting diode (LED) and a fabricating method thereof.
2. Discussion of the Related Art
Recently, InxGa1-xN used for active layers of light emitting diodes (LEDs) using nitride-based compound semiconductors are known materials capable of light emitting over an entire range of visible rays according to configuration of In because ranges of its energy band gap are broad.
Application area of the LEDs is so wide that they are used for electric bulletin boards, display elements, backlight elements, electric bulbs and the like. Such being the case, the application area is gradually expanded to make it imperative to develop high-graded LEDs.
FIG. 1 illustrates a schematic cross-sectional view of an LED according to the conventional techniques, where a sapphire substrate 100 is sequentially stacked thereon by N-GaN layer 101, an active layer 102, P-GaN layer 103. Mesa-etching is performed from the P-GaN layer 103 to portions of the N-GaN layer 101. An upper surface of the P-GaN layer 103 is sequentially formed by a transparent electrode 104 and a P-metal layer 105, and an N-metal layer 106 is formed on the mesa-etched N-GaN layer 101.
The LED thus structured is bonded to a molding cup using adhesive 108, and a first lead frame 109a connected to one external lead and the N-metal layer 103 are wire-bonded, and a second lead frame 109a connected to the other external lead and the P-metal layer 105 are wire-bonded for assembly.
Now, operation of the LEDs thus structured will be explained. If a voltage is applied to a N metal layer and a P metal layer, electrons and holes flow into the active layer 102 from the N-GaN layer 101 and the P-GaN layer 103 to generate the re-coupling of electrons and holes and the light emission.
The light emitted from the active layer 102 advances from thereunder and thereabove, and the light advancing from the above the active layer 102 is discharged outside via the P-GaN layer 103, and portions of light advancing above go downward to escape to the outside of the LED chip, and portions of light escape under a sapphire substrate to be absorbed into or reflected from solder employed for assembling the LED chips, and again advance above the active layer. Portions of the light are again absorbed by the active layer or exit to outside via the active layer.
However, because the aforementioned LED, known as a horizontal LED, is manufactured on a sapphire substrate having a low thermal conductivity, it is difficult to discharge the heat generated in the process of device operation, thereby resulting in problems of degraded characteristics of the devices.
Another problem is that portions of the active layer should be removed for formation of electrodes as shown in FIG. 1, which decreases a light emitting region to make it difficult to realize high luminance and high grade of LEDs, results in decreased number of chips on a same wafer and difficulty in fabricating process, and two-time bondings during assembly.
Yet another problem is that if a sapphire is used as substrate during processes of lapping, polishing, scribing and breaking for dicing-out (separation) of a unit chip following completion of process of LED chips on a wafer, device fabrication yield decreases due to hardness of the sapphire material and non-uniformity of cleavage planes with GaN.
FIGS. 2a to 2h are schematic cross-sectional views explaining a fabrication process of a vertical LED, where the fabrication is processed in such a manner that LED structure is stacked on a sapphire substrate 121 using Metal Oxide Chemical Vapor Deposition (MOCVD), and electrodes and reflector films are formed on an upper surface of the P-GaN 125. The wafer 120 is bonded to a separately manufactured support substrate 130 and the sapphire substrate 121 is removed.
First, referring to FIG. 2a, the MOCVD process is performed on the sapphire substrate 121, and an undoped GaN layer 122, a GaN layer 122, an N-GaN layer 123, InxGa1-xN layer 124 and a P-GaN layer 125 are stacked in sequence.
Successively, the P-GaN layer 125 is sequentially formed at an upper surface thereof with a transparent electrode 126, a reflector film 127, a solder response check layer 128, and a metal layer 129 selected out of any one of Ti/Au, Ni/Au and Pt/Au.
Next, referring to FIG. 2b, first and second ohmic contact metal layers 131 and 132 are respectively formed on the upper and lower surfaces of a base substrate 130 in which electric current can flow.
Successively, the first and second ohmic contact metal layers 131 and 132 are formed thereon with LED chip bonding solder 133 (see FIG. 2c).
Next, the LED chip bonding solder 133 is bonded thereon to the metal layer 129 and a structure formed on the sapphire substrate 121 is bonded to the base substrate 130 (see FIG. 2d).
Successively, laser is irradiated on the sapphire substrate 121 to dice out the sapphire substrate 121 from the undoped GaN layer 122 (see FIG. 2e). The undoped GaN layer 122 remains as a damaged layer up to a predetermined thickness by the Laser Lift Off (LLO) process.
Subsequently, a total etching is performed by using the dry etching process until the N-GaN layer 123 is exposed, and an upper surface of the N-GaN layer 123 corresponding to respective LEDs is formed with an N-electrode pad 141 (see FIG. 2g).
Lastly, cutting processes of scribing and breaking are performed from the second ohmic contact metal layer 132 to the N-GaN layer 123 to dice out into individual devices.
Accordingly, there is an advantage in the LEDs according to the present invention in that it is fabricated in a vertical structure disposed with respective electrodes on the upper and the lower surface thereof, and fabricating process is simplified due to omission of the conventional etching process.
However, the conventional techniques suffer from the following problems. In other words, when the sapphire substrate is separated, a high temperature process is needed because a base substrate for supporting a wafer formed with the LED structure is bonded using solder, such that coefficient of thermal expansion and crystal lattice constant generate stress among different materials, resulting in defects on the LED. As a result, development of a forming process for a support substrate not requiring a condition of the high temperature is much demanded.